The most basic, most widely agreed-upon definition of "indie movie" is any film made mostly without the help of a major studio. Many of them eventually have a studio sign on for distribution, but that doesn’t automatically exclude a movie from being indie. The fact of the matter is that you generally need at least a little nudge from a major movie company if you want the final product to play in theaters or end up on one of the big streaming services.
That being said, as with music, the label of "indie" is sometimes applied to movies based purely on their content, style, or aesthetic. Wes Anderson movies are often described as "indie" even though he’s been working well within the big studio system for most of his career — his films just have that quirky feel and that character-driven vibe that are typically associated with indie filmmaking. Things get further complicated when a lot of money is spent on a movie but it’s the filmmaker’s own money. Mel Gibson largely self-financed "The Passion of the Christ" (per The Hollywood Reporter), so it’s technically an indie movie, but it’s an indie movie that cost over $30 million to make.
When making a list like this, though, some ground rules need to be laid in order to keep things focused. So, for full disclosure, movies that were considered "indie enough" to be eligible for this list had to have their actual production be fairly low budget and be completed with little or no studio involvement. It’s fine if a big studio got on board to distribute or even kick in a little funding on the home stretch, so long as it was primarily just to help the movie as it already existed get finished and released.
50. Primer (2004)
Much was made about how the indie psychological sci-fi film "Primer" was written and directed by an engineer with an advanced math degree who refused to dumb down any of the science and technical jargon in the script. Esquire summed it up thusly: "Anybody who claims he fully understands what’s going on in ‘Primer’ after seeing it just once is either a savant or a liar." Indeed, had Shane Carruth — also one of the movie’s two lead actors — taken his "Primer" script to a major studio, they would’ve forced him to overexplain everything that happens in the movie for a wider audience and completely ruin what sets it apart from the typical sci-fi/time travel movie.
Most time travel movies only work precisely because they lightly dance over the science and the finer details, preventing audiences from thinking about things too much and letting them just enjoy the ride. That "Primer" has the confidence to get down into the nitty gritty of things and not be afraid that the scrutiny will make the whole thing fall apart is astounding. If time travel actually existed, it would create moral conundrums the likes of which we as a species would not be prepared for, and "Primer" is one of the only movies to tackle that aspect of it.